A careful analysis of the cost of operating an automobile was conducted by a firm. The following model was developed: y $4,0000\mathrm{.}20$x$,$ where y the annual cost and x is the miles driven. a$)$ If the car is driven $15,000$ miles this year, the forecasted cost of operating this automobile $($enter your response as a whole number$).$ b$)$ If the car is driven $25,000$ miles this year, the forecasted cost of operating this automobile $($enter your response as a whole number$).$